Best Flooring for a Humid Climate

Q.I'm planning on
installing laminate flooring on the first floor of my North
Carolina house, which is built on a ventilated crawlspace. But
I'm concerned that the laminate finish will trap moisture and
eventually lead to rot damage in the subfloor and framing. Is
laminate flooring a bad choice for this type of house? If so,
what alternative would you recommend?

A.Larry Elkin, a forensic
mechanical engineer with Applied Building Sciences near
Charleston, S.C., and author of several articles about floor
moisture problems in humid climates, responds: Under the
right conditions, any type of flooring that has a relatively
impermeable surface — including laminate flooring, sheet
goods, ceramic tile, and even wood flooring with a thick
polyurethane finish — can act as a cold-side vapor
retarder and lead to the situation you describe.

The problem seems to be more common in colder houses,
particularly when the air conditioning is ducted through
toe-kick registers rather than through wall or ceiling ducts.
We've found fan-shaped patterns of rot damage in front of these
registers, right where very cool supply air blows across the
floor surface; this is caused by condensation, which has the
potential to develop in this location for several months of the
year in warm, humid climates. However, toe-kick registers are
not needed to create this condensation problem. Any home that
is very cool and has excessive crawlspace moisture is subject
to rot.

JLC has published several good articles about drying
and sealing crawlspaces (see "Fixing a Wet Crawlspace," 8/04;
"Building a Sealed Crawlspace," 10/03; and "Crawlspace
Ventilation Update," 8/99). These articles describe strategies
for reducing crawlspace moisture to levels that will not likely
impact the performance of the interior flooring.

But before sealing up the crawlspace, you should consult with
local building-code officials, who may need to be convinced a
sealed crawlspace is an acceptable (or superior) alternative to
a traditional, vented crawlspace; and with a pest-control
expert, who may have concerns about inspection and treatment of
the crawlspace. These details are best worked out before the
project begins.

Finally, be careful about constructing a sealed crawlspace in a
flood-prone area. Besides allowing airflow, crawlspace vents
are often installed to equalize hydrostatic forces across the
foundation, so vent closures must be designed to break away
under flood-induced forces. An alternative may be to seal the
floor assembly at the bottom of the floor joists.